This application claims benefit of Japanese Application No. 2000-378009 filed in Japan on Dec. 12, 2000, the contents of which are incorporated by this reference.
1. Field of the Invention
The present invention relates to a heat sublimatic thermal transfer printer.
2. Description of the Related Art
In recent years, heat-sensitive heat transfer printers have become popular as devices for producing hardcopies of an image received from a personal computer, a camera-inclusive video tape recorder, an electronic still camera, or the like. The heat-sensitive heat transfer printers can reproduce high-definition images owing to the ability to deal with all colors. This type of conventional printer includes the one disclosed in Japanese Unexamined Patent Publication No. 8-310021 (hereinafter referred to as literature 1).
In the conventional heat-sensitive thermal transfer printer, print paper is pressed against and sandwiched between a platen roller and a thermal head together with an ink sheet. The ink sheet has heat sublimable dyes applied over a base film, and is positioned so that the heat sublimable dyes will stick to the print paper. A plurality of heating elements are arranged on one side of the thermal head. When the thermal head is electrically conducted, the heating elements generate heat according to printing data. Consequently, the heat sublimable dyes are heated through the base film, and then sublimated and transferred to the print paper. Printing is thus achieved.
Incidentally, the printing density on print paper is determined by the temperature of the heating elements. In other words, by changing an amount of current to be supplied to the heating elements of the thermal head, the printing density can be changed readily. As a method of changing an amount of current to be delivered to the heating elements, a method of changing a time during which a current is delivered to the heating elements (hereinafter, referred to as a conduction time) is adopted.
By extending the conduction time during which a current is delivered to the heating elements, the density for photo-printing can be raised. However, when the conduction time during which a current is delivered to the heating elements is extended, the time required for printing increases. In particular, as far as a color printer is concerned, printing must be performed four times using, for example, four inks of yellow (Y), magenta (M), cyan (C), and black (BK) as heat sublimable dyes. The extension of the conduction time therefore invites a great increase in the printing time.
In order to shorten the conduction time without narrowing the range of densities for photo-printing, a voltage to be applied to the thermal head is usually set to be relatively high. For example, a conventionally widely adopted voltage to be applied to the thermal head ranges from 22 V to 28 V.
In recent years, various pieces of equipment have come to be portable. The portable equipment is demanded to be more and more compact. The trend to a compact design does not make printers an exception. In order to realize a portable printer, a battery must be used as a power supply.
On the other hand, various types of batteries are known as batteries for portable equipment. In recent years, a lithium-ion secondary cell has been widely adopted because its battery capacity is improved to be large and it is easier to use. Currently, a lithium-ion cell whose rated output voltage is 3.6 V, 3.7 V, or 3.8 V is widely known to suit portable equipment. Assume that this type of lithium-ion cell is used to realize a power supply of a portable printer like the aforesaid one. In this case, since the printer is portable equipment, as long as the number of lithium-ion cells loaded in the printer is four or so, the printer would be realized as a printer whose size satisfies the condition that the printer should be portable. In this case, if four lithium-ion cells whose rated output voltage is 3.6 V, 3.7 V or 3.8 V are juxtaposed, a supply voltage of 14.4 V, 14.8 V, or 15.2 V is developed.
However, conventionally, the portable printer using a thermal head is, as mentioned above, designed on the assumption that a battery is adopted. Moreover, the portable printer requires a supply voltage of approximately 30 V for the purpose of shortening the conduction time. In other words, if the voltage developed from a battery loaded in equipment is 14.4 V, 14.8 V, or 15.2 V, the voltage must be boosted to approximately 30 V.
FIG. 3 is a block diagram showing a power feed circuit for the aforesaid type of thermal head. Referring to FIG. 3, a direct current (dc) power supply 2 develops a power supply voltage of, for example, 14.4 V, 14.8 V, or 15.2 V. The power supply voltage is applied to a DC-DC converter 1 and boosted to approximately 30 V. The voltage boosted by the DC-DC converter 1 is applied to the heating elements of the thermal head through a terminal 3.
As mentioned above, as far as the conventional portable heat sublimatic printer is concerned, the DC-DC converter is adopted for developing a required power supply voltage. However, the DC-DC converter for boosting a dc voltage of 14.4 V, 14.8 V, or 15.2 V into approximately 30 V is very large in size. This poses a problem in that the printer gets large in size. Moreover, the DC-DC converter causes significant power loss. This leads to an increase in power consumption required by a battery. It is therefore impossible to keep driving equipment employing the DC-DC converter for a prolonged period of time.
The literature 1 has proposed that a battery are incorporated as a power supply, and disclosed that a nickel-cadmium cell capable of developing 14.4 V is adopted as the battery. However, the proposal has a drawback that it takes much time to achieve printing as mentioned previously. Moreover, no mention is made of the structure of a thermal head and a control sequence that are suitable for a compact printer and that help improve the performance of the printer. The aforesaid problems are left unsolved.
An object of the present invention is to provide a heat sublimatic printer capable of being designed compactly without the necessity of extending a conduction time by optimizing a supply voltage and resistances offered by heating elements.
Briefly, according to the present invention, there is provided a heat sublimatic printer consisting mainly of:
a battery whose rated voltage is 14.8 V and that is freely attached or detached to or from a housing of the heat sublimatic printer;
a thermal head incorporated in the housing, provided with a plurality of heating elements whose resistances range from 2800 xcexa9 to 3160 xcexa9, and used to print an image on paper according to image data; and
a control circuit, incorporated in the housing, for applying a supply voltage developed from the battery to the thermal head without boosting it, and controlling the timing of electrically conducting the thermal head.
These objects and advantages of the present invention will become further apparent from the following detailed explanation.